DIY Engine Build - well mostly

madjak · Postby **madjak** » Fri Dec 09, 2016 2:02 pm

So far it's been a good discussion about my Rotrex concept, but I don't want it to turn into a turbo vs supercharger debate. Thats been had many times. A turbo will always destroy a Rotrex at low boost. The turbo isn't linked to the RPM's so it can spool and boost at low revs. The issue is always the delay to build boost but that's another discussion.

For me a turbo isn't an option. I don't like the feel as a driver and I can't push the car to the limit on a track with the throttle response and power curve a turbo car delivers. I want the feel of an N/A and I want that linear power delivery. I could drop in a 400whp V8 but I'd rather try what I'm proposing, high reving BP with a Rotrex.

Look at F1, it's taken a long time for the drivers to be able to get in a car and drive them close to the limit. Sure the turbo cars are as fast as the old V10's but they are far harder to drive at the limit. We are also talking about massive development and a full team of engineers modifying throttle maps, tune, energy recovery systems etc etc... something that we can never achieve as a hobbiest. They are also running e-turbos which counter the lag effects of a turbo.

You can do what I'm proposing with a turbo. Dann is probably the one who would best to give advice about it, given how he controls the boost levels to supply a flat torque curve. The issue with pushing that power higher in the revs is that you need a larger turbine in the exhaust so that it's less restrictive to the amount of exhaust flow at high revs. By increasing the turbine size you are making it less effective at lower revs (aka slower spool) so that is the comprimise. Maybe when the e-turbos are available to consumers this would be a good option as the electric drive can counter the slower spool and off throttle response.

The old group B rally cars were set up just like this. Big turbos to generate power at high revs so that the drivetrain could survive, but they were really laggy. It works well on a rally car because the tyres act like a massive clutch, keeping the engine in the power band, and with anti-lag etc to keep the turbo spooled between gears and off throttle. On a track it's less effective as you can't slip the tyres. Sure they are still fast, but it's not the style of car I want as a driver. Fast down the straights and slow through the tight stuff is the opposite of what I want in an MX5... I think with a Rotrex setup I can have both. hopefully.

Alex 2550 · Postby **Alex 2550** » Fri Dec 09, 2016 4:11 pm

this is going to be awesome!
I for one want to see how this turns out as the rotrex interests me, if you pull off what you plan (which i have no doubt from your other projects you will) this will be something special and different.
best of luck with it all.

Alex

madjak · Postby **madjak** » Fri Dec 09, 2016 5:05 pm

datfreak wrote:Your builds are always left field and interesting, I cant wait to see the results of your work- subed.
The cnc chamber work is so good - I've got a spare Na8 head with +1 os values and porting if you want another head to experiment on
Are you going to go with china rods? At one point I was hoping to organise a group buy but iirc the min order was 5 sets at about $250au/set. There are some on ebay for about $300 landed :http://www.ebay.com.au/itm/181287280067

I'm happy to machine the combustion chambers of other heads if anyone needs them done. They issue is shipping them over to Perth and back. It's probably best to wait until I've run this head to see how it goes.

In terms of rods, it's the only thing I'm not really sure on. The issue with all the cheap ebay rods is that they are heavy. It's fine for turbo builds where revs aren't a big issue, but I'm shooting for high revs with this engine and even though the rods themselves won't fail, other parts like the big end bearings might be put under more strain. Actually the bearings will probably be the week point. On my N/A engine I replace the main and big end bearings yearly.

The only good 'I' or'A' beam rods are Carrillo or Crower with Carrillo being the lightest. Maybe I should look at aluminium rods... you can get them custom made fairly cheaply these days. I might run my existing block to save money and just switch heads. I would prefer to build a new bottom end but then I'd rather get the Rotrex earlier.

datfreak wrote:May be a silly irreverent question but does the crank and pulley mating have enough key strength to hold 50-60 hp(plus alt) load for the supercharger?

I don't think that is an issue. The 6-rib belt will be the limiter here rather than the keyway. I have to figure out the damper and belt setup yet... it might be a custom billet part for the SC pulley.

project.r.racing · Postby **project.r.racing** » Fri Dec 09, 2016 5:50 pm

madjak wrote:The old group B rally cars were set up just like this. Big turbos to generate power at high revs so that the drivetrain could survive, but they were really laggy. It works well on a rally car because the tyres act like a massive clutch, keeping the engine in the power band, and with anti-lag etc to keep the turbo spooled between gears and off throttle. On a track it's less effective as you can't slip the tyres. Sure they are still fast, but it's not the style of car I want as a driver. Fast down the straights and slow through the tight stuff is the opposite of what I want in an MX5... I think with a Rotrex setup I can have both. hopefully.

Yes and no.

They still had to use the gears and modulate the throttle to keep the power on. So and turbo lag should be only felt in 1st gear. The rest of the time you are using 4500rpm or higher. Just like a screaming NA car.

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Loving the machined head. The machine is very interesting.